The central hypothesis of this proposal is that the predisposition to breast cancer conferred by a germline mutation in BRCA1 or BRCA2 is subject to modulation by genetic cofactors. The specific aims of the project are: 1) Determine if candidate and/or anonymous single nucleotide polymorphisms (SNPs) are associated with specific germline mutations of BRCA1 ro BRCA2 in Ashkenazim women with breast cancer; 2) Determine if there are differences in frequencies of candidate or anonymous polymorphisms in BRCA-associated breast cancer ascertained on the basis of family history; 3) Determine if the frequency of polymorphisms is different in breast cancer cases with and without BRCA mutations, compared to the frequencies in a reproductive-age sample of individuals of Ashkenazim background; 4) Determine whether the presence of candidate or anonymous polymorphisms in BRCA- associated breast cancer cases correlates with age-at-onset of the disease; and 5) Identify candidate BRA modifier loci utilizing linkage analysis based on informative families ascertained during the course of the study. A two stage strategy will be undertaken utilizing the expanded resources of the Cooperative Family Registry for Breast Cancer Studies ( CFRBCS) and other collaborating centers. Stage one, will genotype 2000 breast cancer cases from clinic-based and population-based ascertainments. This will address specific aims 1,2, and 3. Stage 2 will be an analysis of the frequency of specific polymorphisms in 1000 BRCA mutation-carriers with and without breast cancer. This will address the forth specific aim. Kindreds identified from both stages will provide informative families for the last aim. Analysis will be restricted to a genetically isolated population (Ashkenazim) known to demonstrate linkage disequilibrium at loci of other cancer susceptibility alleles. In addition to SNP's samples will be analyzed for selected large deletions of expansions of genes which have been implicated as putative susceptibility alleles in meta-analyses. Initially, 1494 SNPs will be analyzed. SNPs identified through the Cancer Genome Anatomy Project will be analyzed utilizing both microarrays and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) analysis in the Laboratory of Population Genetics at the NCI. Linkage disequilibrium approaches, will facilitate the identification of loci of potential modifying alleles. The demonstration that the breast cancer risk in BRCA heterozygotes may be modified by other genetic loci would be practical importance in the risk stratification of such women, and, potentially, to target intervention studies to decrease the burden of cancer in this population.